Gas-liquid systems or reactions

More fundamentally, gas-liquid systems promote reaction in the liquid phase by utilizing the following three possible modes of gas-liquid contact 1) gas bubbling through a liquid (as in bubble colunms and stirred tanks) 2) gas contacting a thin film of flowing liquid (as with packed beds or colunms) and 3) liquid droplets dispersed in gas (as in spray colunms and venturi scrubbers). 

Mass transfer in the continuous phase is less of a problem for liquid-liquid systems unless the drops are very small or the velocity difference between the phases is small. In gas-liquid systems, the resistance is always on the liquid side, unless the reaction is very fast and occurs at the interface. The Sherwood number for mass transfer in a system with dispersed bubbles tends to be almost constant and mass transfer is mainly a function of diffusivity, bubble size, and local gas holdup. 

Equilibrium in multiphase and/or multireaction systems. If more than one phase is present in the system, a criterion of phase equilibria has to be satisfied together with the chemical equilibrium criterion. For instance, in a gas-liquid system components are in chemical equilibrium in the phase where the reaction occurs, but vapour-liquid equilibria between the gas and the liquid phases must also be taken into account. To determine the equilibrium composition of a reacting mixture in both phases, chemical equilibrium constants as well as data concerning vapour-liquid equilibria for all components of the reaction mixture should be known. In the equilibrium state ... 

For gas-liquid systems at low mixer speeds, the gas may flow through the reaction liquid resulting in a small interfacial area. At higher mixing rates, the gas bubbles decrease in size, thus enlarging the interfacial area. An increase in gas flow (larger superficial velocity or gas load) may ultimately lead to flooding the reactor [202]. 

In many industrial applications of packed columns, it is desirable to know the volumetric hold-up of the liquid phase in the column. This information might be needed, for example, if the liquid were involved in a chemical reaction or if a control system for the column were being designed. For gas-liquid systems the hold-up of liquid Hw for conditions below the loading point has been found(48) to vary approximately as the 0.6 power of the liquid rate, and for rings and saddles this is given approximately by ... 

The liquid bulk is assumed to be at chemical equilibrium. Contrary to gas-liquid systems, for vapour-liquid systems it is not possible to derive explicit analytical expressions for the mass fluxes which is due to the fact that two or more physical equilibrium constants m, have to be dealt with. This will lead to coupling of all the mass fluxes at the vapour - liquid interface since eqs (15c) and (19) have to be satisfied. For the system described above several simulations have been performed in which the chemical equilibrium constant K = koiAo2 and the reaction rate constant koi have been varied. Parameter values used in the simulations are given in Table 5. The results are presented in Figs 9 and 10. 

Henry s law equilibrium. If there is no reaction in the liquid phase (or it is slow relative to uptake and diffusion), the gas-liquid system eventually comes to equilibrium, which can usually be described by Henry s law discussed earlier. This does not reflect a lack of uptake of the gas at equilibrium but rather equal rates of uptake and evaporation i.e., it is a dynamic equilibrium (see Problem 12). The equilibrium between the gas-and liquid-phase concentrations is characterized by the Henry s law constant, H (mol L-1 atm-1), where II = [X /I. ... 

Consider first the main characteristic features of formation of the layers of chemical compounds, common to solid-solid, solid-liquid and solid-gas systems (Chapters 1 to 4). Then, the effect of dissolution of a solid in the liquid phase of a solid-liquid system or of its evaporation into the gaseous phase of a solid-gas system on the growth kinetics of a chemical compound layer will be analysed in Chapter 5. Thus, under the conditions of occurrence of a chemical reaction its product will be assumed to be solid and to form a continuous compact layer adherent at least to one of the initial phases. 

The gas-liquid interfacial area measured form the physical techniques is generally about 35% higher than the one measured by the chemical technique. The chemical technique is generally more accurate than the physical technique. The reaction systems described in Table XXXI are reliable for the gas-liquid system. For gas-liquid-solid systems, the method is not reliable even when solids are inert, because of possible adsorptions of gas and/or liquid on the solid surface. 

Many processes of gas absorption with chemical reaction are set up at high pressures, result of technical and/or economical requirements. That is, for example, processes of hydrocracking and hydrorefining of heavy oils and processes of oxydation of liquid effluents. However, if many chemical systems are found to determine the mass transfer parameters in an industrial reactor at atmospheric pressure by using the chemical method, they become scarce at elevated pressures. Several physical and chemical methods have been proposed chemical methods present some severe drawbacks, since one has to replace the gas-liquid system of interest by another one, presenting different physical properties (specially a different coalescence behaviour). 

Recommendations All existing literature data on gas and liquid holdups for countercurrent flow systems are for packings normally used in absorption towers or gas -liquid reactors. The validity of these data for small packings that would normally be used in gas-liquid-solid catalytic reactions needs to be checked. 

Gas-liquid systems are classified as coalescing or noncoalescing, depending on the behavor exhibited by the bubbles. Water is classed as coalescing by Konig (7). Examples of systems exhibiting lower coalescence rates are 0.5% propanol solutions, dilute solutions of surfactants and many of the media used in fermentation reactions. 

For a volatile liquid reactant or a volatile product, these steps are essentially reversed. For a nonvolatile liquid reactant or product, only the reaction and diffusion in the liquid take place. Figure 7-15 describes the absorbing gas concentration profiles in a gas-liquid system. 

It remains to be noted, that particularly large quantities of gas can then be dispersed with the smallest stirrer power with turbine stirrers, if D/d < 3andh/d < 0.5. In addition, however, under these conditions the gas bubbles remain longer than usual in the liquid and for rapid chemical reactions in gas/liquid systems this can possibly lead to depletion of the gaseous reaction component (e.g. oxygen or air). 

The physical aspects of gas-liquid, liquid-liquid, solid-liquid, and gas-liquid-solid systems are discussed in the subsequent sections of this chapter. Using the guidelines given there, it is possible to get an estimate of local and average mass-transfer coefficients, interfacial areas, and contacting patterns. This section briefly considers the effect of reactions on mass transfer, a subject treated elsewhere in this handbook and in advanced texts [29, 30]. Note that while we will refer mostly to gas-liquid systems, the same treatment would more or less apply to liquid-liquid and liquid-solid systems. In the case of gas-liquid-solid systems, it may be possible to determine the controlling resistance and simplify the analysis to a two-phase system, as far as the reaction part is concerned. 

---